Vertically polarised omnidirectional antenna

ABSTRACT

A waveguide having a cross-section of oblong shape is pierced with two arrays of longitudinal slots located opposite one another, symmetrically in relation to the longitudinal plane of symmetry of the guide intersecting the cross-section thereof along its smaller dimension. In order to obtain the desired omnidirectional pattern, which requires that the distance between oppositely located slots should not be too great, a waveguide is used the greater dimension of the cross-section of which is reduced relatively to the value required by the operation frequency, and ridges or dielectric material are provided inside the guide for increasing its cut-off frequency.

The present invention relates to an omnidirectional antenna, forradiating a vertically polarised wave, designed in the form of a slottedwaveguide.

Omnidirectional antennas of this kind which are used in the centimeterwaveband, have the advantage over antennas of the dipole array kind,that they need nothing more than a simple wave guide feeder instead ofthe complex kind of feeder system which otherwise has to be used inorder to take account of each radiator element.

It is well-known to design omnidirectional antennas of this kind in theform of circular waveguides pierced by a network of slots.

The drawback of these omnidirectional antennas based upon circularwaveguides, arises out of the fact that it is not a very easy matter toproduce in a circular waveguide a propagation mode which will yieldvertical polarisation when the latter is required.

The object of the present invention is an omnidirectional antenna forradiating a vertically polarised wave which combines the inherent feederarrangement simplicity of antennas formed from circular waveguides, withthe relative ease of design of antennas constituted by dipole arrays.

This result is achieved using slotted waveguide with an oblongcross-section, and additional elements.

It is further known, for example from U.S. Pat. No. 2,543,468, to designomnidirectional antennas for radiating a horizontally polarised wave byutilising waveguides having an oblong cross-section with a symmetry axisin the direction of its greater dimension; for that purpose thewaveguides are pierced, in the direction of their length, with twoarrays of longitudinal slots located opposite one another, symmetricallyin relation to that one of the longitudinal symmetry planes of thewaveguide which contains the above-mentioned symmetry axis. It wouldappear, at first sight, that, similarly, a vertically polarisedomnidirectional waveguide antenna with an oblong cross-section could beobtained provided the cross-section of the waveguide shows a symmetryaxis in the direction of its smaller dimension and the waveguide ispierced, in the direction of its length, with two arrays of transversalslots located opposite one another, symmetrically in relation to thatone of the longitudinal symmetry planes containing the symmetry axisshown by the cross-section of the waveguide in the direction of itssmaller dimension. As a matter of fact such antennas have not beendesigned because opposite slots are at a distance from each other whichis equal to the greater dimension of the cross-section of the waveguideand so the elementary sources constituted by two opposite slots are at adistance from each other which is too great, relatively to the operationwave length, for the antenna to be considered as omnidirectional.

According to the invention there is provided an omnidirectional antennafor radiating a vertically polarised wave, comprising a waveguide withan oblong cross-section and a longitudinal plane of symmetry which cutssaid cross-section in the direction of the smaller dimension thereof,said waveguide having a wall provided with two arrays of slots locatedopposite one another and arranged symmetrically in relation to saidplane of symmetry; and a device located inside said waveguide forincreasing the cut-off frequency thereof; and two identical metalplates, attached to the exterior of said waveguide and located in saidplane of symmetry and extending over the whole length of said waveguide.

The invention will be better understood and other of its featuresrendered apparent from a consideration of the ensuing description andthe related drawings in which:

FIGS. 1 and 2 are fragmentary views of antennas in accordance with theinvention;

FIGS. 3 and 4 are diagrams which explain the operation of an antenna inaccordance with the invention.

FIG. 1 illustrates an antenna designed to produce an omnidirectionalpattern in the azimuthal plane, with a vertically polarised wave. Thisantenna comprises a rectangular waveguide 1 with radiator slots such as2 3 and 4. These slots have a perimeter substantially equal to thewavelength λ of the wave to be transmitted by the waveguide 1, and aredistributed in the form of two identical arrays, located opposite oneanother in the shorter sides of the waveguide, the longer sides of theslots in one and the same array being parallel with one another.Consecutive slots at one and the same side are arranged at variableintervals (for example at intervals ranging λ_(g) /4 and 2 λ_(g), whereλ_(g) is the wavelength in the waveguide), this because of thedistribution, in the waveguide 1, of the current lines which said slotsmust intersect in order to perform the function of radiation elements.Moreover, the oppositely disposed slots are arranged in order to radiatein phase.

Inside the waveguide 1 there are disposed two metal ridges 5, 6. Theridges extend over the whole length of the waveguide and arerespectively attached to the two longer internal faces, at the centresthereof; the object of these ridges is to increase the cut-off frequencyof the waveguide and thus allow the use of a waveguide having longersides shorter than would be otherwise necessary and therefore makingoppositely located slots sufficiently close to each other for theobtainment of an omnidirectional pattern.

At the centres of the longer sides of the ridged waveguide section,there are respectively attached two metal plates 7, 8, which extend overthe whole length of the waveguide; the function of these plates is tocorrect the radiation patterns of the waveguide slots in order topromote the production of an omnidirectional pattern.

FIG. 2 illustrates an antenna which differs from that of FIG. 1 only inthat a dielectric material 9 which fills the waveguide is substitutedfor the ridges. This material plays the same part as the ridges: toallow to reduce the dimensions of the greater side of the waveguidewithout reducing the cut-off frequency thereof.

In this case, as in the case of FIG. 1, the characteristics of theantenna remain the same:

opposite slots radiating in phase;

distance between opposite slots of the order of half the operatingwavelength λ;

slot perimeter substantially equal to the operating wavelength λ;

variable distance between two consecutive slots of one and the samearray (for example distance ranging between λ_(g) /4 and 2 λ_(g) , whereλ_(g) is the wavelength in the waveguide).

It should be noted that in order to simplify matters, the slots havebeen shown in FIGS. 1 and 2, perpendicular to the major axis of thewaveguide 1, although in reality the slots can be inclined to a greateror lesser extent, in order to modify the coupling between slot andwaveguide and to bring the coupling factor to a level which contributesto the formation of the desired overall pattern.

FIG. 3 illustrates, in a horizontal plane defined by two rectangularaxes Ox and Oy, elementary patterns of two elementary sources S₁ and S₂whose respective co-ordinates are (-d/2, 0) and (+d/2, O), these sourcesnot radiating to any substantial extent towards the rear, and radiatingin phase.

If θ is the angle made by a given direction with the axis Oy and F₁ (θ)and F₂ (θ) the elementary patterns of the sources, then calculationshows that the total pattern F_(t), in the case where the two sourcesare spaced by an interval corresponding to half a wavelength (d = λ/ 2),is given by: ##EQU1## The two elementary sources being chosen identical,##EQU2##

The graph representing F_(t) as a function of θ is given in FIG. 4.

The two sources each contribute half to the radiation in the direction θ= 0° or θ = π, while a single source S₁, yields the radiation at -π/2,and a single source S₂, the radiaton at + π2; thus, in order to have thesame amplitude in the directions O, π, - π2, and + π/2, it is necessaryfor the elementary pattern of a source to exhibit a drop of 6 dB betweenits peak value, that is to say as the case may be θ = - π/2 or θ + π/2,and its value for θ = 0° or θ = π. It is as a function of theserequirements that the precise position and direction of each slot willbe determined.

It should be noted, moreover, that experiment, confirmed by calculation,has shown that the phase along the omnidirectional pattern, variesalternately between 0° and π/2.

An antenna such as that described in relation to FIG. 1, has been builtfor a transmitter operating at a frequency of 5000 MHz. The antennameasures around 2m in height; the longer side of the waveguide is 30 mm.the shorter side 20 mm; the ridges have a thickness of 2 mm and arespaced apart by 10 mm; the plates have a width of 30 mm.

The radiation pattern can be perfected, although at the expense ofsimplicity by utilising a waveguide which, such as the rectangularsection waveguide, has an oblong cross-section and which enables thesame modes of propagation as those occuring in a rectangular sectionwaveguide to exist, while making it possible to form slots on curvedsurfaces; it may be, for example, a waveguide of ellipticalcross-section or a waveguide whose cross-section exhibits two straightand parallel sides separated by a distance shorter than their own lengthand linked together by two arcuate portions.

Of course, the invention is not limited to the embodiments described andshown which were given solely by way of example.

What is claimed is:
 1. An omnidirectional antenna for radiating avertically polarised wave, comprising: a waveguide with an oblongcross-section and a longitudinal plane of symmetry which cuts saidcross-section in the direction of the smaller dimension thereof, saidwaveguide having a wall provided with two arrays of slots locatedopposite one another and arranged symmetrically in relation to saidplane of symmetry; a device located inside said waveguide for increasingthe cut-off frequency thereof; and two identical metal plates, attachedto the exterior of said waveguide and extending over the whole lengththereof, said plates being located in said plane of symmetry.